Semiconductor device and method for manufacturing the same

ABSTRACT

A bottom-gate transistor with a short channel length and a method for manufacturing the transistor are provided. A bottom-gate transistor with a short channel length in which portions of a source electrode and a drain electrode which are proximate to a channel formation region are thinner than other portions thereof was devised. In addition, the portions of the source electrode and the drain electrode which are proximate to the channel formation region are formed in a later step than the other portions thereof, whereby a bottom-gate transistor with a short channel length can be manufactured.

TECHNICAL FIELD

The present invention relates to a semiconductor device typified by atransistor and a method for manufacturing the semiconductor device.

BACKGROUND ART

Transistors in which a semiconductor material is deposited over asubstrate having an insulating surface to be used as an active layer(hereinafter referred to as deposited-film transistors) have beenresearched. A silicon-based semiconductor material such as amorphoussilicon has been conventionally used as an active layer; however,attention is focused in recent years on research on transistors in whichan oxide semiconductor material is used as an active layer. This isbecause a transistor in which an oxide semiconductor material is used asan active layer (hereinafter referred to as an oxide semiconductortransistor) has higher on-state current and lower off-state current thana transistor in which amorphous silicon is used as an active layer.

Further, there have been attempts to develop a semiconductor devicewhich functions as a memory or the like by forming an oxidesemiconductor transistor having the above features in a layer differentfrom a transistor including single crystal silicon, and the like (PatentDocument 1 and Non-Patent Document 1). In the structure of such asemiconductor device, the transistor in the upper layer is preferably abottom-gate transistor. This is because a wiring which electricallyconnects the transistor in the lower layer can serve also as a gateelectrode of the transistor in the upper layer.

REFERENCES Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2011-238333

Non-Patent Document

-   [Non-Patent Document 1] K. Kaneko et al., “Highly Reliable    BEOL-Transistor with Oxygen-controlled InGaZnO and Gate/Drain Offset    Design for High/Low Voltage Bridging I/O Operations”, IEDM 2011, pp.    155-158.

DISCLOSURE OF INVENTION

In the above semiconductor device, which includes a plurality oftransistors in a plurality of layers, the bottom-gate transistor formedin the upper layer is preferably a deposited-film transistor. This isbecause an active layer can be easily formed by deposition, whichfacilitates manufacture of the semiconductor device.

Conventional semiconductor devices which include a plurality oftransistors in a plurality of layers and in which a bottom-gatetransistor is formed in the upper layer are easy to manufacture, but donot have sufficient performance capabilities as semiconductor devices.This is because the bottom-gate transistor in the upper layer does nothave sufficient electric characteristics. For example, in a memory usinga semiconductor device which includes a plurality of transistors in aplurality of layers, a transistor which performs writing to the memorydoes not have sufficient writing capability or the like when thetransistor is a deposited-film transistor. This is because thebottom-gate transistor in the upper layer does not have sufficientelectric characteristics, particularly because the on-state current islower than that of a transistor in which bulk silicon is used as anactive layer. Therefore, it is necessary to increase the on-statecurrent of the deposited-film transistor. One method for achieving thisis to use the bottom-gate transistor whose channel length is reduced(for example, to about 30 nm). Note that a photolithography processusing an electron beam is necessary for reducing the channel length toless than 30 nm.

In order to reduce the channel length of a bottom-gate transistor whosesource electrode and drain electrode are formed by separating oneconductive layer using a photolithography process, the thickness of aresist needs to be less than or equal to the desired channel length.Meanwhile, the thickness of the resist is reduced during the etchingstep of the conductive layer. Therefore, the thickness of the conductivelayer needs to be set such that the conductive layer can be separatedbefore the resist is removed in the etching step.

On the other hand, there is a limitation on reduction in the thicknessof the conductive layer because the source electrode and the drainelectrode of the transistor preferably have low electric resistance.

As described above, it is difficult to reduce the channel length of thebottom-gate transistor while suppressing the electric resistance betweenthe source electrode and the drain electrode.

An object of one embodiment of the present invention is to provide abottom-gate transistor with a short channel length. Another object is toprovide a method for manufacturing a bottom-gate transistor with a shortchannel length.

The inventors have focused on the structure of the source electrode andthe drain electrode of the bottom-gate transistor. Thus, the inventorshave devised a structure in which portions of the source electrode andthe drain electrode which are proximate to a channel formation regionare thinner than other portions thereof.

The inventors have also devised a method in which the portions of thesource electrode and the drain electrode which are proximate to thechannel formation region are formed in a later step than the otherportions thereof.

Further, the inventors focused on the following phenomenon: steps whichare caused between a portion proximate to the channel formation regionand the other portions by the formation of the other portions (i.e.,portions other than the portions proximate to the channel formationregion) of the source electrode and the drain electrode cannot becovered with a resist at the time of the formation of the portions ofthe source electrode and the drain electrode which are proximate to thechannel formation region. Thus, the inventors have devised a method formanufacturing a bottom-gate transistor with a short channel length,which includes the steps of: covering the steps with an insulatinglayer, planarizing the insulating layer, forming a hard mask layer overthe planarized insulating layer, and separating the portion proximate tothe channel formation region with the hard mask layer.

Specifically, one embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of: forming agate electrode layer over an insulating surface; forming a gateinsulating layer on and in contact with the gate electrode layer;forming an oxide semiconductor layer overlapping with the gate electrodelayer on and in contact with the gate insulating layer; forming aconductive layer covering the oxide semiconductor layer on and incontact with the oxide semiconductor layer; forming, on and in contactwith the conductive layer, a first low-resistance material layer and asecond low-resistance material layer apart from each other with the gateelectrode layer provided therebetween; forming a first protective layeron and in contact with the first low-resistance material layer, thesecond low-resistance material layer, and the conductive layer;planarizing the first protective layer; forming a hard mask layer on andin contact with the planarized first protective layer; forming, on asurface of the hard mask layer, a resist pattern including an openingpattern portion in a region that is between the first low-resistancematerial layer and the second low-resistance material layer and overlapswith the oxide semiconductor layer; etching the hard mask layer usingthe resist pattern to form an opening pattern; etching the firstprotective layer using the hard mask layer including the opening patternas a mask until the conductive layer is exposed; etching the conductivelayer using the hard mask layer and the first protective layer includingthe opening pattern as masks to separate the conductive layer into afirst conductive layer and a second conductive layer; and filling anopening in the first protective layer with a second protective layer.

The steps between the portion proximate to the channel formation regionand the other portions are planarized with an insulating layer, andthen, a hard mask layer is formed and a resist for forming an opening isapplied on the hard mask layer. Since the surface on which the resist isto be applied is flat, the resist can be uniformly formed, whichprevents a region which cannot be covered with the resist from beingformed and enables the resist to be thinly and uniformly formed. Thus,an opening pattern with a small line width can be formed with the resistover the hard mask layer.

As described above, a transistor with a short channel length can beformed by processing a conductive layer with the use of a hard masklayer. Even when the resist is removed during the processing, a problemin that processing cannot be performed after the removal is preventedowing to the hard mask layer. This is because the hard mask layer servesas a mask for the processing of the first protective layer and theconductive layer. Note that the hard mask layer can be formed using afilm which is not easily etched under conditions for etching the firstprotective layer and the conductive layer.

Through the above steps, the conductive layer for forming the sourceelectrode and the drain electrode can be provided with an opening with aminute pattern. Thus, a bottom-gate transistor with a short channellength can be manufactured.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of: forming agate electrode layer over an insulating surface; forming a gateinsulating layer on and in contact with the gate electrode layer;forming an oxide semiconductor layer overlapping with the gate electrodelayer on and in contact with the gate insulating layer; forming aconductive layer covering the oxide semiconductor layer on and incontact with the oxide semiconductor layer; forming a low-resistancematerial layer on and in contact with the conductive layer; forming awiring protective layer on and in contact with the low-resistancematerial layer; forming a first wiring protective layer and a secondwiring protective layer apart from each other with the gate electrodelayer provided therebetween by processing the wiring protective layer;forming a first low-resistance material layer and a secondlow-resistance material layer apart from each other with the gateelectrode layer provided therebetween by processing the low-resistancematerial layer, the first low-resistance material layer being in contactwith the first wiring protective layer and the second low-resistancematerial layer being in contact with the second wiring protective layer;forming a resist pattern including an opening pattern portion in aregion that is between the first low-resistance material layer and thesecond low-resistance material layer and overlaps with the oxidesemiconductor layer; etching the conductive layer using the resistpattern to separate the conductive layer into a first conductive layerand a second conductive layer; and filling an opening in the conductivelayer with a protective layer.

Through the above steps, the conductive layer for forming the sourceelectrode and the drain electrode can be provided with an opening with aminute pattern without the low-resistance material layer being removedduring the processing of the conductive layer. Thus, a bottom-gatetransistor with a short channel length can be manufactured.

In the method for manufacturing a semiconductor device according to oneembodiment of the present invention, the thickness of the low-resistancematerial layer is not reduced and the surface of the low-resistancematerial layer is not damaged during the processing of the conductivelayer for forming the source electrode and the drain electrode.Therefore, the wiring resistance of the low-resistance material layer isnot increased. The low-resistance material layer can be used as a wiringfor electrically connecting the transistor to another semiconductorelement. Thus, an integrated circuit which includes a semiconductordevice manufactured by the method can operate at high speed becausewiring delay due to high wiring resistance hardly occurs.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer; a gate insulating layer on and incontact with the gate electrode layer; an oxide semiconductor layerbeing on and in contact with the gate insulating layer and overlappingwith the gate electrode layer; a first conductive layer and a secondconductive layer provided on and in contact with the oxide semiconductorlayer apart from each other with the gate electrode layer providedtherebetween; a first low-resistance material layer on and in contactwith the first conductive layer; a second low-resistance material layeron and in contact with the second conductive layer; a first protectivelayer on and in contact with the first conductive layer, the firstlow-resistance material layer, the second conductive layer, and thesecond low-resistance material layer; and a second protective layer incontact with a part of the oxide semiconductor layer. In thesemiconductor device, a distance between the first conductive layer andthe second conductive layer is shorter than a distance between the firstlow-resistance material layer and the second low-resistance materiallayer. The first conductive layer and the first low-resistance materiallayer serve as a source electrode and the second conductive layer andthe second low-resistance material layer serve as a drain electrode.

When the above structure is applied to a bottom-gate transistorincluding an oxide semiconductor layer, the transistor can have a shortchannel length and thus can have high on-state current. Further, sincean oxide semiconductor has higher electron mobility than amorphoussilicon, a semiconductor device with high on-state current can beprovided.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer; a gate insulating layer on and incontact with the gate electrode layer; an oxide semiconductor layerbeing on and in contact with the gate insulating layer and overlappingwith the gate electrode layer; a first conductive layer and a secondconductive layer provided on and in contact with the oxide semiconductorlayer apart from each other with the gate electrode layer providedtherebetween; a first low-resistance material layer on and in contactwith the first conductive layer; a second low-resistance material layeron and in contact with the second conductive layer; a first wiringprotective layer on and in contact with the first low-resistancematerial layer; a second wiring protective layer on and in contact withthe second low-resistance material layer; and a protective layer on andin contact with the first conductive layer, the first wiring protectivelayer, the second conductive layer, and the second wiring protectivelayer and in contact with a part of the oxide semiconductor layer. Inthe semiconductor device, a distance between the first conductive layerand the second conductive layer is shorter than a distance between thefirst low-resistance material layer and the second low-resistancematerial layer. The first conductive layer and the first low-resistancematerial layer serve as a source electrode and the second conductivelayer and the second low-resistance material layer serve as a drainelectrode.

When the above structure is applied to a bottom-gate transistorincluding an oxide semiconductor layer, the transistor can have a shortchannel length and thus can have high on-state current. Further, sincean oxide semiconductor has higher electron mobility than amorphoussilicon, a semiconductor device with high on-state current can beprovided.

The ends of the wiring protective layers on the channel side might berounded through the processing for forming the conductive layers. Inthat case, the coverage with the protective layer can be favorable ascompared with the case where the ends are not rounded. The protectivelayer functions as a passivation film; thus, better coverage with theprotective layer enables moisture or the like to be further preventedfrom entering from the outside. This is particularly effective in atransistor using an oxide semiconductor, whose electric characteristicsare easily affected by moisture or the like which enters from theoutside.

In any of the above semiconductor devices, the gate insulating layer ispreferably flat.

Planarization of a base insulating layer and the gate electrode layercan prevent non-coverage of the oxide semiconductor layer due to a stepcaused by the gate electrode layer. The planarization is particularlyeffective when the thickness of the oxide semiconductor layer is greaterthan or equal to 5 nm and less than or equal to 30 nm.

In any of the above semiconductor devices, a width of the oxidesemiconductor layer in a channel length direction is preferably largerthan a width of the gate electrode layer in the channel lengthdirection.

In that case, the oxide semiconductor layer overlaps with the gateelectrode layer in a large area, which enables the oxide semiconductorlayer to be supplied with oxygen more easily from an insulating layerbelow the oxide semiconductor layer. As a result, the initial electriccharacteristics (e.g., threshold voltage) and reliability in electriccharacteristics (e.g., threshold voltage) of the transistor can beimproved.

Moreover, oxygen vacancies are likely to be formed in the end of anisland-shaped oxide semiconductor layer, and therefore carriers are morelikely to be generated than in other regions thereof. Local generationof carriers in the oxide semiconductor layer, which is an active layer,causes degradation of electric characteristics (e.g., threshold voltage)of the transistor.

Suppose that the width of the oxide semiconductor layer in the channellength direction is smaller than the width of the gate electrode layerin the channel length direction, that is, the end of the island-shapedoxide semiconductor layer is on the inner side than the end of the gateelectrode layer, an electric field is concentrated on the end of theisland-shaped oxide semiconductor layer upon application of a voltagebetween the gate electrode layer and the source electrode. Concentrationof an electric field on the end of the island-shaped oxide semiconductorlayer, where carriers are likely to be generated, leads to degradationof electric characteristics (e.g., threshold voltage) of the transistor.On the other hand, when the width of the oxide semiconductor layer inthe channel length direction is larger than the width of the gateelectrode layer in the channel length direction as in one embodiment ofthe present invention, the end of the island-shaped oxide semiconductorlayer is on the outer side than the end of the gate electrode layer;thus, an electric field is not concentrated on the end of theisland-shaped oxide semiconductor layer upon application of a voltagebetween the gate electrode layer and the source electrode. This cansuppress degradation of electric characteristics (e.g., thresholdvoltage) of the transistor.

According to one embodiment of the present invention, a bottom-gatetransistor whose channel length is short (e.g., 30 nm) and in which anoxide semiconductor layer is used as an active layer can bemanufactured. Further, a semiconductor device including the transistoras a component can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are a plan view and cross-sectional views illustratingone embodiment of the present invention.

FIGS. 2A-1 to 2A-3, 2B-1 to 2B-3, and 2C-1 to 2C-3 are plan views andcross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 3A-1 to 3A-3, 3B-1 to 3B-3, and 3C-1 to 3C-3 are plan views andcross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 4A-1 to 4A-3, 4B-1 to 4B-3, and 4C-1 to 4C-3 are plan views andcross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 5A-1 to 5A-3, 5B-1 to 5B-3, and 5C-1 to 5C-3 are plan views andcross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 6A-1 to 6A-3, 6B-1 to 6B-3, and 6C-1 to 6C-3 are plan views andcross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 7A and 7B are a plan view and a cross-sectional view illustratingone embodiment of the present invention.

FIG. 8 is a circuit diagram illustrating one embodiment of the presentinvention.

FIGS. 9A to 9C are a plan view and cross-sectional views illustratingone embodiment of the present invention.

FIGS. 10A-1 to 10A-3, 10B-1 to 10B-3, and 10C-1 to 10C-3 are plan viewsand cross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 11A-1 to 11A-3, 11B-1 to 11B-3, and 11C-1 to 11C-3 are plan viewsand cross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 12A-1 to 12A-3 and 12B-1 to 12B-3 are plan views andcross-sectional views illustrating one embodiment of the presentinvention.

FIGS. 13A and 13B are a plan view and a cross-sectional viewillustrating one embodiment of the present invention.

FIGS. 14A and 14B illustrate an example of a memory device.

FIG. 15 illustrates an example of a memory device.

FIG. 16 illustrates an example of a memory device.

FIGS. 17A to 17C illustrate an example of an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device which canbe manufactured according to the present invention will be describedwith reference to FIGS. 1A to 1C. FIG. 1A is a plan view of a transistor440, FIG. 1B is a cross-sectional view taken along line A-A′ in FIG. 1A,and FIG. 1C is a cross-sectional view taken along line B-B′ in FIG. 1A.A channel length L of the transistor 440 is greater than or equal to 20nm and less than or equal to 100 nm, preferably greater than or equal to20 nm and less than or equal to 50 nm, more preferably greater than orequal to 20 nm and less than or equal to 30 nm. In this embodiment, thechannel length L is about 30 nm.

The transistor 440 in FIGS. 1A to 1C is a bottom-gate transistor. Thetransistor 440 illustrated in FIGS. 1A to 1C includes, over a baseinsulating layer 436 formed over a surface of a substrate 400, a gateelectrode layer 401 provided so as to be buried in an insulating layer432, a gate insulating layer 402 over the gate electrode layer 401, anoxide semiconductor layer 403 over the gate insulating layer 402, afirst conductive layer 454 a and a second conductive layer 454 b overthe oxide semiconductor layer 403, a first low-resistance material layer405 a on and in contact with the first conductive layer 454 a, a secondlow-resistance material layer 405 b on and in contact with the secondconductive layer 454 b, a first protective layer 406 in contact with thefirst low-resistance material layer 405 a, the second low-resistancematerial layer 405 b, the first conductive layer 454 a, and the secondconductive layer 454 b, a hard mask layer 495 in contact with the firstprotective layer 406, and a second protective layer 407 over the hardmask layer 495.

First, components will be described below.

<Components of the Semiconductor Device> (Substrate and Base InsulatingLayer)

As the substrate 400, a substrate having an insulating surface can beused; a substrate having at least heat resistance high enough towithstand heat treatment in a later step is preferable. For example, aglass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like can be used as the substrate 400. A singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like; a compoundsemiconductor substrate made of silicon germanium or the like; an SOIsubstrate; or the like can be used as the substrate 400, or thesubstrate provided with a semiconductor element can be used as thesubstrate 400. Note that the concentration of impurities such ashydrogen or water in the substrate 400 is preferably low. This is forpreventing diffusion of hydrogen or water into the oxide semiconductorlayer 403 so as to prevent degradation of the electric characteristicsof the semiconductor device.

As the base insulating layer 436, an oxide insulating layer of siliconoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, or thelike or a nitride insulating layer of silicon nitride, silicon nitrideoxide, aluminum nitride, aluminum nitride oxide, or the like can beused, for example.

(Gate Electrode Layer)

The gate electrode layer 401 can be formed using a metal material suchas molybdenum, titanium, tungsten, aluminum, or copper, for example.Alternatively, a semiconductor layer typified by a polycrystallinesilicon layer doped with an impurity element such as phosphorus, or asilicide layer such as a nickel silicide layer may be used as the gateelectrode layer 401. The gate electrode layer 401 may have either asingle-layer structure or a stacked-layer structure.

(Gate Insulating Layer)

The gate insulating layer 402 can be formed using silicon oxide, siliconoxynitride, silicon nitride, or the like. The gate insulating layer 402is preferably a silicon oxide layer containing oxygen in a proportionhigher than that in the stoichiometric composition. The gate insulatinglayer 402 may be a single layer or a stack of two layers formed usingany of the above materials. For example, silicon nitride and siliconoxynitride, or silicon nitride and silicon oxide can be used.

(Source Electrode Layer and Drain Electrode Layer)

One of the source electrode layer and the drain electrode layer includesthe first conductive layer 454 a and the first low-resistance materiallayer 405 a, and the other of the source electrode layer and the drainelectrode layer includes the second conductive layer 454 b and thesecond low-resistance material layer 405 b. The first conductive layer454 a and the second conductive layer 454 b can be formed using a metalsuch as tungsten or molybdenum. Tungsten is especially preferred becausethe ratio of the etching rate of the first conductive layer 454 a andthe second conductive layer 454 b to that of the first protective layer406 can be increased. A stack of aluminum and titanium, copper, or thelike can be used for the first low-resistance material layer 405 a andthe second low-resistance material layer 405 b. The stack of aluminumand titanium may be titanium, aluminum, and titanium stacked in thisorder. In the case of using copper for the first low-resistance materiallayer 405 a and the second low-resistance material layer 405 b, titaniumnitride or the like is preferably provided to prevent diffusion ofcopper into an adjacent layer.

(Semiconductor Layer)

A semiconductor that has a band gap wider than at least that of silicon,1.1 eV, can be used for a semiconductor layer of a transistor accordingto one embodiment of the present invention; an oxide semiconductor ispreferably used. In this embodiment, the case where the oxidesemiconductor layer 403 is used as the semiconductor layer is described.

The thickness of the oxide semiconductor layer 403 is greater than orequal to 5 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 30 nm. The thickness is setsuch that the channel length of the transistor is reduced whileshort-channel effect is suppressed.

A material that can be used as the oxide semiconductor contains at leastindium (In). In particular, In and zinc (Zn) are preferably contained.As a stabilizer for reducing variation in electric characteristics of atransistor using the oxide semiconductor, gallium (Ga) is preferablyadditionally contained. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) ispreferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, any of the following can be used: indiumoxide, tin oxide, zinc oxide, an In—Zn-based oxide, an In—Mg-basedoxide, an In—Ga-based oxide, an In—Ga—Zn-based oxide (also referred toas IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, anIn—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide,an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-basedoxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, anIn—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide,an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-basedoxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than the In, Ga, and Zn.

A material represented by InMO₃(ZnO)_(m) (m>0, where m is not aninteger) may be used as the oxide semiconductor. Note that M representsone or more metal elements selected from Ga, Fe, Mn, and Co.Alternatively, a material represented by In₂SnO₅(ZnO)_(n) (n>0, where nis an integer) may be used as the oxide semiconductor.

As the oxide semiconductor, an In—Ga—Zn-based oxide with an atomic ratioof In:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), orIn:Ga:Zn=3:1:2 (=1/2:1/6:1/3), or an oxide with an atomic ratio close tothe above atomic ratios can be used. Alternatively, an In—Sn—Zn-basedoxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3),In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or anoxide with an atomic ratio close to the above atomic ratios may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used as the oxide semiconductorcontaining indium depending on needed semiconductor characteristics(e.g., electric characteristics such as mobility and threshold voltage,and variation in the electric characteristics). In order to obtain theneeded semiconductor characteristics, it is preferable that the carrierconcentration, the impurity concentration, the defect density, theatomic ratio between a metal element and oxygen, the interatomicdistance, the density, and the like be set to appropriate values.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn-based oxide. However, mobility can be increased byreducing the defect density in a bulk also in the case of using anIn—Ga—Zn-based oxide.

The oxide semiconductor layer 403 is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

It is preferable that the oxide semiconductor layer 403 be highlypurified so as to hardly contain impurities such as copper, aluminum, orchlorine. In the process for manufacturing the transistor, steps inwhich these impurities are not mixed or attached to the surface of theoxide semiconductor layer 403 are preferably selected as appropriate. Inthe case where the impurities are attached to the surface of the oxidesemiconductor layer 403, the impurities on the surface of the oxidesemiconductor layer 403 are preferably removed by exposure to oxalicacid, dilute hydrofluoric acid, or the like or by plasma treatment (suchas N₂O plasma treatment). Specifically, the copper concentration in theoxide semiconductor is lower than or equal to 1×10¹⁸ atoms/cm³,preferably lower than or equal to 1×10¹⁷ atoms/cm³. In addition, thealuminum concentration in the oxide semiconductor is lower than or equalto 1×10¹⁸ atoms/cm³. Further, the chlorine concentration in the oxidesemiconductor is lower than or equal to 2×10¹⁸ atoms/cm³.

The oxide semiconductor immediately after being deposited is preferablyin a supersaturated state in which the proportion of oxygen is higherthan that in the stoichiometric composition. For example, in the casewhere the oxide semiconductor is deposited using a sputtering method,the deposition is preferably performed under the condition where theproportion of oxygen in a deposition gas is high, in particular, in anoxygen atmosphere (oxygen gas: 100%). When the deposition is performedunder the condition where the proportion of oxygen in the deposition gasis high, particularly in a 100% oxygen gas atmosphere, release of Znfrom the film can be suppressed even at a deposition temperature higherthan or equal to 300° C., for example.

The oxide semiconductor is preferably supersaturated with oxygen bysufficient removal of impurities such as hydrogen followed by sufficientsupply with oxygen. Specifically, the hydrogen concentration in theoxide semiconductor is lower than or equal to 5×10¹⁹ atoms/cm³,preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferablylower than or equal to 5×10¹⁷ atoms/cm³. Note that the above hydrogenconcentration in the oxide semiconductor is measured by secondary ionmass spectrometry (SIMS).

(Protective Layer)

The first protective layer 406 has a function of protecting the oxidesemiconductor layer 403 from entry of moisture or the like from theoutside. The first protective layer 406 can be formed using a siliconoxide film, a silicon oxynitride film, or an aluminum oxide film. Thefirst protective layer 406 is preferably formed using a silicon oxidefilm, a silicon oxynitride film, or an aluminum oxide film containingmuch oxygen. In order to make the first protective layer contain muchexcess oxygen, oxygen may be added as appropriate to the firstprotective layer by an ion implantation method, an ion doping method, orplasma treatment.

The second protective layer 407 has a function of protecting the oxidesemiconductor layer 403 from entry of moisture or the like from theoutside. The second protective layer 407 can be formed using a siliconoxide film, a silicon oxynitride film, or an aluminum oxide film. Thesecond protective layer 407 is preferably formed using a silicon oxidefilm, a silicon oxynitride film, or an aluminum oxide film containingmuch oxygen. Alternatively, the second protective layer 407 may includetwo layers: a third protective layer 407 a and a fourth protective layer407 b. The third protective layer 407 a is formed in contact with theoxide semiconductor. The third protective layer 407 a can be formedusing an oxide semiconductor film containing gallium (Ga), a siliconoxide film, a silicon oxynitride film, or an aluminum oxide film whichis deposited with conditions set as appropriate so as to contain muchoxygen. The fourth protective layer 407 b is formed in contact with thethird protective layer 407 a. The fourth protective layer 407 b can beformed using a silicon oxide film, a silicon oxynitride film, or analuminum oxide film containing much oxygen. In order to make the secondprotective layer contain much excess oxygen, oxygen may be added asappropriate to the second protective layer by an ion implantationmethod, an ion doping method, or plasma treatment.

(Hard Mask Layer)

The hard mask layer 495 is preferably a film that is not easily etchedunder conditions for etching the first protective layer 406. This isbecause the hard mask layer 495 is used as a mask in the etching of thefirst protective layer 406. The hard mask layer 495 is preferably formedusing amorphous silicon, which can be deposited by a PCVD method or asputtering method.

<Method for Manufacturing Semiconductor Device>

A method for manufacturing the semiconductor device according to oneembodiment of the present invention is described with reference to FIGS.2A-1 to 2A-3, 2B-1 to 2B-3, and 2C-1 to 2C-3; FIGS. 3A-1 to 3A-3, 3B-1to 3B-3, and 3C-1 to 3C-3; FIGS. 4A-1 to 4A-3, 4B-1 to 4B-3, and 4C-1 to4C-3; FIGS. 5A-1 to 5A-3, 5B-1 to 5B-3, and 5C-1 to 5C-3; and FIGS. 6A-1to 6A-3, 6B-1 to 6B-3, and 6C-1 to 6C-3.

FIGS. 2A-1 to 2A-3, 2B-1 to 2B-3, and 2C-1 to 2C-3 illustrate the stepsfrom formation of the gate electrode layer 401 to oxygen doping of theoxide semiconductor layer 403.

First, the substrate 400 is prepared, the base insulating layer 436 isformed over the substrate 400, and the gate electrode layer 401 isformed over the base insulating layer 436 (see FIGS. 2A-1 to 2A-3).

The gate electrode layer 401 is formed by depositing a material that canbe used as a gate electrode using sputtering, for example, and thenselectively etching part of the material. Note that the etching may bedry etching, wet etching, or both dry etching and wet etching. Thesubstrate 400 and the gate electrode layer 401 may be subjected to heattreatment after the formation of the gate electrode layer 401.

Next, the insulating layer 432 is formed over the base insulating layer436 and the gate electrode layer 401. The insulating layer 432 ispreferably planarized so that the gate electrode layer 401 is exposedand the insulating layer 432 and the gate electrode layer 401 areplanarized (see FIGS. 2B-1 to 2B-3). As the planarization treatment,chemical mechanical polishing (CMP) treatment or the like may beperformed.

The insulating layer 432 and the gate electrode layer 401 are preferablyplanarized so that non-coverage of the oxide semiconductor layer 403,which is described later, due to a step caused by the gate electrodelayer 401 can be prevented.

Next, the gate insulating layer 402 is formed over the gate electrodelayer 401, and the oxide semiconductor layer 403 is formed over the gateinsulating layer 402 (see FIGS. 2C-1 to 2C-3).

For example, for the gate insulating layer 402, a film of a materialwhich can be used for the gate insulating layer 402 can be formed by aPCVD method.

Note that before the oxide semiconductor layer 403 is formed, heattreatment may be performed for dehydration or dehydrogenation of thegate insulating layer 402. For example, heat treatment may be performedat a temperature higher than or equal to 350° C. and lower than or equalto 450° C.

In addition, oxygen may be supplied to the gate insulating layer 402which has been dehydrated or dehydrogenated. Oxygen may be contained inthe gate insulating layer 402, or in the gate insulating layer 402 andin the vicinity of an interface of the gate insulating layer 402. By thesupply of oxygen to the gate insulating layer 402 after dehydration ordehydrogenation, the release of oxygen can be suppressed and theconcentration of oxygen in the gate insulating layer 402 can beincreased. Oxygen can be supplied by oxygen doping treatment or thelike.

Note that in the case of performing heat treatment for supplying oxygenfrom the gate insulating layer 402 to the oxide semiconductor, the heattreatment is preferably performed before the oxide semiconductor isprocessed into an island shape, in which case oxygen contained in thegate insulating layer 402 can be prevented from being released by theheat treatment.

For example, the heat treatment is performed at a temperature higherthan or equal to 350° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 350° C. and lower than orequal to 450° C. Heat treatment may be further performed in a laterstep. As a heat treatment apparatus for the heat treatment, for example,an electric furnace or an apparatus for heating an object by heatconduction or heat radiation from a heater such as a resistance heatercan be used; for example, a rapid thermal annealing (RTA) apparatus suchas a gas rapid thermal annealing (GRTA) apparatus or a lamp rapidthermal annealing (LRTA) apparatus can be used.

Further, after the heat treatment is performed and while the heatingtemperature is being maintained or being decreased, a high-purity oxygengas, a high-purity N₂O gas, or ultra-dry air (having a dew point of −40°C. or lower, preferably −60° C. or lower) may be introduced into thefurnace where the heat treatment has been performed. It is preferablethat the oxygen gas or the N₂O gas do not contain water, hydrogen, andthe like. The purity of the oxygen gas or the N₂O gas which isintroduced into the heat treatment apparatus is preferably 6N or higher,further preferably 7N or higher; that is, the impurity concentration inthe oxygen gas or the N₂O gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower. By the action of the oxygen gas or the N₂Ogas, oxygen is supplied to the oxide semiconductor, and defects due tooxygen vacancies in the oxide semiconductor can be reduced. Note thatthe high-purity oxygen gas, high-purity N₂O gas, or ultra-dry air may beintroduced during the heat treatment.

Further, oxygen doping 451 is performed on the oxide semiconductor (seeFIGS. 2C-1 to 2C-3). This is for supplying oxygen to the oxidesemiconductor to fill oxygen vacancies in the oxide semiconductor.Filling the oxygen vacancies makes the semiconductor device less likelyto exhibit abnormal initial electric characteristics (e.g., thresholdvoltage) and improves its reliability in electric characteristics (e.g.,threshold voltage).

The oxygen doping 451 can be performed by an ion implantation method, anion doping method, a plasma immersion ion implantation method, plasmatreatment, or the like. By use of such methods, the oxide semiconductorcan be doped with oxygen (an oxygen radical, an oxygen atom, an oxygenmolecule, ozone, an oxygen ion (an oxygen molecular ion), and/or anoxygen cluster ion).

FIGS. 3A-1 to 3A-3, 3B-1 to 3B-3, and 3C-1 to 3C-3 illustrate the stepsfrom processing of the oxide semiconductor layer 403 into an islandshape to formation of a resist 453 for forming the first low-resistancematerial layer 405 a and the second low-resistance material layer 405 b.

The oxide semiconductor layer 403 is processed into an island-shapedoxide semiconductor layer 403 by a photolithography step (see FIGS. 3A-1to 3A-3).

The etching of the oxide semiconductor layer 403 may be dry etching, wetetching, or both dry etching and wet etching.

Note that the width of the oxide semiconductor layer in the channellength direction is preferably larger than the width of the gateelectrode layer in the channel length direction. In that case, the oxidesemiconductor layer overlaps with the gate electrode layer in a largearea, which enables the oxide semiconductor layer to be supplied withoxygen more easily from an insulating layer below the oxidesemiconductor layer. As a result, the initial electric characteristics(e.g., threshold voltage) and reliability in electric characteristics(e.g., threshold voltage) of the transistor can be improved.

Next, a conductive layer 454 is formed in contact with the oxidesemiconductor layer 403. The conductive layer 454 may be formed by asputtering method or the like (see FIGS. 3B-1 to 3B-3).

Then, a low-resistance material layer 405 is formed in contact with theconductive layer 454. The low-resistance material layer 405 may beformed by a sputtering method or the like.

Next, the resist 453 is formed by a photolithography step (see FIGS.3C-1 to 3C-3).

FIGS. 4A-1 to 4A-3, 4B-1 to 4B-3, and 4C-1 to 4C-3 illustrate the stepsfrom processing of the low-resistance material layer 405 toplanarization of the first protective layer 406.

The low-resistance material layer 405 is selectively etched using theresist 453 as a mask; thus, the low-resistance material layer 405 a andthe low-resistance material layer 405 b are formed (see FIGS. 4A-1 to4A-3). The conditions for etching the low-resistance material layer 405are set such that the conductive layer 454 is not easily etched. This isbecause an opening is formed in the conductive layer 454 in a later stepwith the use of the hard mask layer 495 as a mask.

Next, a region of the conductive layer 454 which is not in contact withthe oxide semiconductor layer 403 is etched (see FIGS. 4B-1 to 4B-3).

Then, the first protective layer 406 is formed and planarized by CMP(see FIGS. 4C-1 to 4C-3). The surface of the first protective layer 406is planarized so that, even when a resist as thin as about 30 nm isapplied thereon, no region is formed which cannot be covered with theresist owing to a step on the surface where the resist is to be applied.

FIGS. 5A-1 to 5A-3, 5B-1 to 5B-3, and 5C-1 to 5C-3 illustrate the stepsfor forming the hard mask layer 495.

The hard mask layer 495 is formed over the planarized first protectivelayer 406 (see FIGS. 5A-1 to 5A-3). The hard mask layer 495 ispreferably a film that is not easily etched under conditions for etchingthe first protective layer 406. This is because the hard mask layer 495is used as a mask in the etching of the first protective layer 406.

Next, a resist is formed over the hard mask layer 495 and subjected toexposure to an electron beam; thus, a resist 455 is formed (see FIGS.5B-1 to 5B-3).

Here, the thickness of the resist is preferably set such that the ratioof the thickness of the resist to the width of the manufactured patternbecomes 1:1 to 1:2. For example, in the case where the width of thepattern is 30 nm, the thickness of the resist is set within 30 nm to 60nm.

The surface of the hard mask layer 495 is flat. Therefore, even a resistas thin as about 30 nm can be uniformly applied on the surface where theresist is to be applied.

Next, the hard mask layer 495 is etched (see FIGS. 5C-1 to 5C-3). Theetching method is preferably dry etching. The resist may be removedafter the hard mask layer 495 is etched. In this embodiment, the hardmask layer 495 includes an opening which overlaps with a channelformation region of the oxide semiconductor layer 403.

FIGS. 6A-1 to 6A-3, 6B-1 to 6B-3, and 6C-1 to 6C-3 illustrate the stepsfrom formation of an opening in the first protective layer 406 toformation of an opening in the conductive layer 454.

The first protective layer 406 is etched (see FIGS. 6A-1 to 6A-3). Theconditions for etching the first protective layer 406 are preferably setsuch that the ratio of the etching rate of the first protective layer406 to that of the hard mask layer 495 is high. This is for forming anopening with a width (width in the channel length direction) of about 30nm in the first protective layer 406 with the use of the hard mask layer495 as a mask.

Next, the conductive layer 454 is etched, so that the first conductivelayer 454 a and the second conductive layer 454 b are formed. A channelis to be formed between the first conductive layer 454 a and the secondconductive layer 454 b (see FIGS. 6B-1 to 6B-3). The conditions foretching the conductive layer 454 are preferably set such that the ratioof the etching rate of the conductive layer 454 to that of the oxidesemiconductor layer 403 is high. This is for preventing the surface ofthe oxide semiconductor layer 403 from being damaged by the etching.

In the case of forming an opening with a width (width in the channellength direction) of about 30 nm, the resist 455 is as thin as 30 nm to60 nm. Therefore, the resist 455 is removed during the etching of thefirst protective layer 406 and the conductive layer 454. However, sincethe hard mask layer 495 serves as a mask, an opening with a width ofabout 30 nm can be formed in the conductive layer 454 even though theresist 455 is removed.

Next, the opening formed in the conductive layer 454 in the above stepis covered with the second protective layer 407 (see FIGS. 6C-1 to6C-3). A film which can prevent entry of moisture, hydrogen, or the likeinto the oxide semiconductor layer 403 is preferably used as the secondprotective layer 407. For example, a silicon oxide film, a siliconoxynitride film, a silicon nitride film, an aluminum oxide film, or thelike can be used.

The second protective layer 407 preferably contains excess oxygen. Theas-deposited second protective layer 407 may contain excess oxygen, orthe second protective layer 407 may be subjected to oxygen doping. Forexample, the doping with oxygen (an oxygen radical, an oxygen atom, anoxygen molecule, ozone, an oxygen ion (an oxygen molecular ion), and/oran oxygen cluster ion) can be performed by an ion implantation method,an ion doping method, a plasma immersion ion implantation method, plasmatreatment, or the like. For the ion implantation method, a gas clusterion beam may be used.

After the second protective layer 407 is formed, heat treatment may beperformed. For example, heat treatment is performed at 250° C. for onehour in a nitrogen atmosphere.

Through the above steps, the transistor 440 can be manufactured. Thechannel length L of the transistor 440 manufactured at that time isabout 30 nm. Therefore, the transistor 440 can have high on-statecurrent.

The above is the method for manufacturing the semiconductor deviceaccording to one embodiment of the present invention.

Through the above method for manufacturing a semiconductor device, abottom-gate transistor in which the oxide semiconductor layer 403 isused as an active layer and whose channel length is about 30 nm can bemanufactured.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device will bedescribed with reference to FIGS. 7A and 7B. FIG. 7A is a plan view of atransistor 420, and FIG. 7B is a cross-sectional view taken along lineX-Y in FIG. 7A. Note that in FIG. 7A, some components of the transistor420 (e.g., the gate insulating layer 402) are not illustrated forsimplicity. A channel length L of the transistor 420 is greater than orequal to 20 nm and less than or equal to 100 nm, preferably greater thanor equal to 20 nm and less than or equal to 50 nm, more preferablygreater than or equal to 20 nm and less than or equal to 30 nm. In thisembodiment, the channel length L is about 30 nm.

The transistor 420 illustrated in FIGS. 7A and 7B includes, over thebase insulating layer 436 formed over a surface of the substrate 400,the gate electrode layer 401 provided so as to be buried in theinsulating layer 432, the gate insulating layer 402 over the gateelectrode layer 401, the oxide semiconductor layer 403 over the gateinsulating layer 402, the first conductive layer 454 a and the secondconductive layer 454 b over the oxide semiconductor layer 403, the firstlow-resistance material layer 405 a on and in contact with the firstconductive layer 454 a, the second low-resistance material layer 405 bon and in contact with the second conductive layer 454 b, the firstprotective layer 406 in contact with the first low-resistance materiallayer 405 a, the second low-resistance material layer 405 b, the firstconductive layer 454 a, and the second conductive layer 454 b, the hardmask layer 495 in contact with the first protective layer 406, and thesecond protective layer 407 over the hard mask layer 495.

Embodiment 1 can be referred to for the structure and manufacturingmethod of the semiconductor device described in this embodiment.

(Circuit which can be Provided Over the Substrate 400)

The substrate 400 is provided with a semiconductor element, which is notillustrated here for simplicity. Wiring layers 474 a and 474 b and thebase insulating layer 436 covering the wiring layers 474 a and 474 b areprovided over the substrate 400, part of which is a component of amemory illustrated in FIG. 8. FIG. 8 illustrates an example of anequivalent circuit, showing a connection between the transistor 420 anda transistor 431 formed using the substrate 400.

A capacitor 430 is a capacitor in which the source electrode layer orthe drain electrode layer, which includes stacked layers, and the wiringlayer 474 a serve as a pair of electrodes and the base insulating layer436 and the gate insulating layer 402, which includes stacked layers,serve as a dielectric.

In the memory configuration illustrated in FIG. 8, writing to the memoryis performed by injection of charge into the capacitor 430. Thetransistor described in this embodiment has a channel length about 30nm, and thus has high on-state current. Therefore, writing to the memorycan be performed at high speed.

The memory configuration illustrated in FIG. 8 has the advantages ofbeing capable of holding stored data even when not powered and having anunlimited number of write cycles. This is because, since the transistordescribed in this embodiment has low off-state current, the chargestored in the capacitor 430 is not easily released.

Embodiment 3

In this embodiment, one embodiment of a semiconductor device which canbe manufactured according to the present invention will be describedwith reference to FIGS. 9A to 9C. FIG. 9A is a plan view of a transistor441, FIG. 9B is a cross-sectional view taken along line A-A′ in FIG. 9A,and FIG. 9C is a cross-sectional view taken along line B-B′ in FIG. 9A.Note that in FIG. 9A, some components of the transistor 441 (e.g., thegate insulating layer 402) are not illustrated for simplicity. A channellength L of the transistor 441 is greater than or equal to 20 nm andless than or equal to 100 nm, preferably greater than or equal to 20 nmand less than or equal to 50 nm, more preferably greater than or equalto 20 nm and less than or equal to 30 nm. In this embodiment, thechannel length L is about 30 nm.

The transistor 441 in FIGS. 9A to 9C is a bottom-gate transistor. Thetransistor 441 illustrated in FIGS. 9A to 9C includes the substrate 400,the base insulating layer 436, the insulating layer 432, the gateelectrode layer 401, the gate insulating layer 402, the oxidesemiconductor layer 403, the first conductive layer 454 a, the secondconductive layer 454 b, the first low-resistance material layer 405 a,the second low-resistance material layer 405 b, a first wiringprotective layer 485 a, a second wiring protective layer 485 b, and thefirst protective layer 406.

The base insulating layer 436 is provided in contact with a surface ofthe substrate 400. The insulating layer 432 is in contact with the baseinsulating layer 436. The gate electrode layer 401 is buried in theinsulating layer 432. The gate insulating layer 402 is provided on andin contact with the gate electrode layer 401. The oxide semiconductorlayer 403 is provided on and in contact with the gate insulating layer402. The first conductive layer 454 a and the second conductive layer454 b are provided on and in contact with the oxide semiconductor layer403. The first low-resistance material layer 405 a is provided on and incontact with the first conductive layer 454 a. The second low-resistancematerial layer 405 b is provided on and in contact with the secondconductive layer 454 b. The first wiring protective layer 485 a isprovided on and in contact with the first low-resistance material layer405 a. The second wiring protective layer 485 b is provided on and incontact with the second low-resistance material layer 405 b. The firstprotective layer 406 is provided in contact with the first conductivelayer 454 a, the second conductive layer 454 b, the first wiringprotective layer 485 a, the second wiring protective layer 485 b, andthe oxide semiconductor layer 403.

First, components will be described below.

<Components of the Semiconductor Device>

Embodiment 1 can be referred to for details of the substrate, the baseinsulating layer, the gate electrode layer, the gate insulating layer,the source electrode layer, the drain electrode layer, the oxidesemiconductor layer, and the protective layer.

(Wiring Protective Layer)

A wiring protective layer 485 is preferably a film that is not easilyetched under conditions for etching the conductive layer 454. The firstlow-resistance material layer 405 a and the second low-resistancematerial layer 405 b are used as lead wirings for connectingtransistors; therefore, the low-resistance material layer 405 has athickness greater than or equal to 100 nm. This means that thedifference in height between the surface of the low-resistance materiallayer 405 and the surface of the conductive layer 454 is at leastgreater than or equal to 100 nm. Without the wiring protective layer485, the end of the low-resistance material layer 405 is not coveredwith a resist, or covered with a thin resist even if covered; thus, thelow-resistance material layer 405 is damaged by the etching forprocessing the conductive layer 454. In order to prevent that damage,the wiring protective layer 485 which is not easily etched underconditions for etching the conductive layer 454 is provided over thelow-resistance material layer 405 to prevent the low-resistance materiallayer 405 from being etched. The wiring protective layer 485 ispreferably formed using silicon oxide, silicon nitride, siliconoxynitride, or aluminum oxide. Silicon oxide, silicon nitride, andsilicon oxynitride can be deposited by a PCVD method or a sputteringmethod. Aluminum oxide can be deposited by a sputtering method.

<Method for Manufacturing Semiconductor Device>

A method for manufacturing the semiconductor device according to oneembodiment of the present invention is described with reference to FIGS.2A-1 to 2A-3, 2B-1 to 2B-3, and 2C-1 to 2C-3; FIGS. 10A-1 to 10A-3,10B-1 to 10B-3, and 10C-1 to 10C-3; FIGS. 11A-1 to 11A-3, 11B-1 to11B-3, and 11C-1 to 11C-3; and FIGS. 12A-1 to 12A-3 and 12B-1 to 12B-3.

FIGS. 2A-1 to 2A-3, 2B-1 to 2B-3, and 2C-1 to 2C-3 illustrate the stepsfrom formation of the gate electrode layer 401 to oxygen doping of theoxide semiconductor layer 403. Embodiment 1 can be referred to for thesesteps.

FIGS. 10A-1 to 10A-3, 10B-1 to 10B-3, and 10C-1 to 10C-3 illustrate thesteps from processing of the oxide semiconductor layer 403 into anisland shape to formation of the resist 453 for forming thelow-resistance material layer 405 and the wiring protective layer 485.

The oxide semiconductor layer 403 is processed into an island-shapedoxide semiconductor layer 403 by a photolithography step (see FIGS.10A-1 to 10A-3). Refer to Embodiment 1 for details.

Next, the conductive layer 454 is formed in contact with the oxidesemiconductor layer 403. The conductive layer 454 may be formed by asputtering method or the like. Then, the low-resistance material layer405 is formed in contact with the conductive layer 454. Thelow-resistance material layer 405 may be formed by a sputtering methodor the like. Then, the wiring protective layer 485 is formed in contactwith the low-resistance material layer 405. The wiring protective layer485 may be formed by a sputtering method or the like (see FIGS. 10B-1 to10B-3).

Next, the resist 453 is formed by a photolithography step (see FIGS.10C-1 to 10C-3).

FIGS. 11A-1 to 11A-3, 11B-1 to 11B-3, and 11C-1 to 11C-3 illustrate thesteps from processing of the wiring protective layer 485 and thelow-resistance material layer 405 to formation of the resist 455 forforming the first conductive layer 454 a and the second conductive layer454 b.

The wiring protective layer 485 and the low-resistance material layer405 are selectively etched using the resist 453 as a mask; thus, thefirst wiring protective layer 485 a, the second wiring protective layer485 b, the first low-resistance material layer 405 a, and the secondlow-resistance material layer 405 b are formed (see FIGS. 11A-1 to11A-3). The wiring protective layer 485 and the low-resistance materiallayer 405 may be processed using the same resist pattern, or may beprocessed by forming their respective resist patterns. The wiringprotective layer 485 and the low-resistance material layer 405 arepreferably etched under conditions such that the conductive layer 454 isnot easily etched.

Next, a region of the conductive layer 454 which is not in contact withthe oxide semiconductor layer 403 is etched (see FIGS. 11B-1 to 11B-3).

Next, a resist is formed over the conductive layer 454, the first wiringprotective layer 485 a, and the second wiring protective layer 485 b.Here, the thickness of the resist is preferably set such that the ratioof the thickness of the resist to the width of the manufactured patternbecomes 1:1 to 1:2. For example, in the case where the width of thepattern is 30 nm, the thickness of the resist is set within 30 nm to 60nm. The resist is subjected to exposure to an electron beam, so that theresist 455 is formed (see FIGS. 11C-1 to 11C-3).

The first low-resistance material layer 405 a and the secondlow-resistance material layer 405 b can be used as lead wirings forconnecting transistors. High wiring resistance of a lead wiring causes aproblem of wiring delay in an integrated circuit; thus, the wiringresistance needs to be lowered. Therefore, the low-resistance materiallayer 405 needs a thickness greater than or equal to 100 nm in general.This means that the difference in height between the surface of thelow-resistance material layer 405 and the surface of the conductivelayer 454 is at least greater than or equal to 100 nm. Without thewiring protective layer 485 over the low-resistance material layer 405,the ends of the first low-resistance material layer 405 a and the secondlow-resistance material layer 405 b are not covered with a resist, orcovered with a thin resist even if covered; thus, the firstlow-resistance material layer 405 a and the second low-resistancematerial layer 405 b are etched at the time of processing the conductivelayer 454. However, in the manufacturing method according to oneembodiment of the present invention, the wiring protective layer 485which is not easily etched under conditions for etching the conductivelayer 454 is provided on and in contact with the low-resistance materiallayer 405, so that the first low-resistance material layer 405 a and thesecond low-resistance material layer 405 b are not reduced in thicknessand the surfaces thereof are not damaged in the processing of theconductive layer 454; thus, an increase in wiring resistance isprevented. Therefore, wiring delay is less likely to occur in anintegrated circuit which includes a semiconductor device manufactured bythe manufacturing method.

FIGS. 12A-1 to 12A-3 and 12B-1 to 12B-3 illustrate the steps fromprocessing of the conductive layer 454 to formation of the firstprotective layer 406.

The conductive layer 454 is etched using the resist 455 as a mask, sothat the first conductive layer 454 a and the second conductive layer454 b are formed. A channel is to be formed between the first conductivelayer 454 a and the second conductive layer 454 b (see FIGS. 12A-1 to12A-3).

The conditions for etching the conductive layer 454 are preferably setsuch that the ratio of the etching rate of the conductive layer 454 tothat of the oxide semiconductor layer 403 is high. This is forpreventing the surface of the oxide semiconductor layer 403 from beingdamaged by the etching.

In the case of forming an opening with a width of about 30 nm, theresist 455 is as thin as 30 nm to 60 nm; thus, a region which cannot becovered with a resist may be formed at the ends of the first wiringprotective layer 485 a and the second wiring protective layer 485 b, forexample. Accordingly, there may be a region where the resist 455 isremoved during the etching of the conductive layer 454 or a region whichis etched without being covered with a resist. However, in a regionwhich is less likely to be covered with a resist, for example, at theends of the first wiring protective layer 485 a and the second wiringprotective layer 485 b, the first low-resistance material layer 405 aand the second low-resistance material layer 405 b are not etched evenwhen the resist 455 is removed because the first wiring protective layer485 a and the second wiring protective layer 485 b protect thelow-resistance material layers.

Next, the opening formed in the conductive layer 454 in the above stepis covered with the first protective layer 406 (see FIGS. 12B-1 to12B-3). A film which can prevent entry of moisture, hydrogen, or thelike into the oxide semiconductor layer 403 is preferably used as thefirst protective layer 406. For example, a silicon oxide film, a siliconoxynitride film, a silicon nitride film, an aluminum oxide film, or thelike can be used.

The first protective layer 406 preferably contains excess oxygen. Forexample, the first protective layer 406 is preferably formed using asilicon oxide film, a silicon oxynitride film, or an aluminum oxide filmcontaining much oxygen. Alternatively, the first protective layer 406may include two layers. The first layer provided in contact with theoxide semiconductor can be formed using an oxide semiconductor filmcontaining gallium (Ga), a silicon oxide film, a silicon oxynitridefilm, or an aluminum oxide film which is deposited with conditions setas appropriate so as to contain much oxygen. The second layer can beformed using a silicon oxide film, a silicon oxynitride film, or analuminum oxide film containing much oxygen. In order to make the firstprotective layer 406 contain much excess oxygen, oxygen may be added asappropriate to the first protective layer 406 by an ion implantationmethod, an ion doping method, or plasma treatment.

After the first protective layer 406 is formed, heat treatment may beperformed. For example, heat treatment is performed at 250° C. for onehour in a nitrogen atmosphere.

Through the above steps, the transistor 441 can be manufactured. Thechannel length L of the transistor 441 manufactured at that time isabout 30 nm. Therefore, the transistor 441 can have high on-statecurrent.

The above is the method for manufacturing the semiconductor deviceaccording to one embodiment of the present invention.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 4

In this embodiment, one embodiment of a semiconductor device will bedescribed with reference to FIGS. 13A and 13B. FIG. 13A is a plan viewof a transistor 421, and FIG. 13B is a cross-sectional view taken alongline X-Y in FIG. 13A. Note that in FIG. 13A, some components of thetransistor 421 (e.g., the gate insulating layer 402) are not illustratedfor simplicity. A channel length L of the transistor 421 is greater thanor equal to 20 nm and less than or equal to 100 nm, preferably greaterthan or equal to 20 nm and less than or equal to 50 nm, more preferablygreater than or equal to 20 nm and less than or equal to 30 nm. In thisembodiment, the channel length L is about 30 nm.

The transistor 421 illustrated in FIGS. 13A and 13B includes thesubstrate 400, the base insulating layer 436, the insulating layer 432,the gate electrode layer 401, the gate insulating layer 402, the oxidesemiconductor layer 403, the first conductive layer 454 a, the secondconductive layer 454 b, the first low-resistance material layer 405 a,the second low-resistance material layer 405 b, the first wiringprotective layer 485 a, the second wiring protective layer 485 b, andthe first protective layer 406.

The base insulating layer 436 is provided in contact with a surface ofthe substrate 400. The insulating layer 432 is in contact with the baseinsulating layer 436. The gate electrode layer 401 is buried in theinsulating layer 432. The gate insulating layer 402 is provided on andin contact with the gate electrode layer 401. The oxide semiconductorlayer 403 is provided on and in contact with the gate insulating layer402. The first conductive layer 454 a and the second conductive layer454 b are provided on and in contact with the oxide semiconductor layer403. The first low-resistance material layer 405 a is provided on and incontact with the first conductive layer 454 a. The second low-resistancematerial layer 405 b is provided on and in contact with the secondconductive layer 454 b. The first wiring protective layer 485 a isprovided on and in contact with the first low-resistance material layer405 a. The second wiring protective layer 485 b is provided on and incontact with the second low-resistance material layer 405 b. The firstprotective layer 406 is provided in contact with the first conductivelayer 454 a, the second conductive layer 454 b, the first wiringprotective layer 485 a, the second wiring protective layer 485 b, andthe oxide semiconductor layer 403.

Embodiment 3 can be referred to for the structure and manufacturingmethod of the semiconductor device described in this embodiment.

Embodiment 5

In this embodiment, a c-axis aligned crystalline oxide semiconductor(CAAC-OS) film, which can be used as the oxide semiconductor layer 403exemplified in Embodiments 1 and 3, will be described.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts are includedin an amorphous phase. Note that in most cases, the crystal part fitsinside a cube whose one side is less than 100 nm. From an observationimage obtained with a transmission electron microscope (TEM), a boundarybetween an amorphous part and a crystal part in the CAAC-OS film is notclear. Further, with the TEM, a grain boundary in the CAAC-OS film isnot found. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is suppressed.

In each of the crystal parts included in the CAAC-OS film, the c-axis isaligned in a direction perpendicular to a surface where the CAAC-OS filmis formed or a surface of the CAAC-OS film, triangular or hexagonalatomic arrangement which is seen from the direction perpendicular to thea-b plane is formed, and metal atoms are arranged in a layered manner ormetal atoms and oxygen atoms are arranged in a layered manner when seenfrom the direction perpendicular to the c-axis. Note that, among crystalparts, the directions of the a-axis and the b-axis of one crystal partmay be different from those of another crystal part. In thisspecification and the like, a simple term “perpendicular” includes arange from 85° to 95°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction perpendicular to the surface where the CAAC-OSfilm is formed or a surface of the CAAC-OS film, the directions of thec-axes may be different from each other depending on the shape of theCAAC-OS film (the cross-sectional shape of the surface where the CAAC-OSfilm is formed or the cross-sectional shape of the surface of theCAAC-OS film). Note that when the CAAC-OS film is formed, the directionof the c-axis of the crystal part is the direction perpendicular to thesurface where the CAAC-OS film is formed or the surface of the CAAC-OSfilm. The crystal part is formed by deposition or by performingtreatment for crystallization such as heat treatment after deposition.

In an oxide semiconductor having a crystal part such as the CAAC-OS,defects in the bulk can be further reduced and when the surface flatnessof the oxide semiconductor is improved, mobility higher than that of anoxide semiconductor in an amorphous state can be obtained. In order toimprove the surface flatness, the oxide semiconductor is preferablyformed over a flat surface. Specifically, the oxide semiconductor may beformed over a surface with an average surface roughness (R_(a)) of lessthan or equal to 1 nm, preferably less than or equal to 0.3 nm, furtherpreferably less than or equal to 0.1 nm. Since the transistor 440 is abottom-gate transistor, it is possible to improve the planarity of thesurface where the oxide semiconductor layer 403 is to be formed byperforming planarization treatment such as CMP treatment after theformation of the gate electrode layer 401 and the base insulating layer436 to obtain the above flat surface.

When the CAAC-OS film is used as the oxide semiconductor layer 403 in atransistor, change in electric characteristics (e.g., threshold voltage)of the transistor due to irradiation with visible light or ultravioletlight can be reduced. Thus, the transistor has high reliability.

Embodiment 6

In this embodiment, an example of a semiconductor device (memory device)which includes the transistor described in this specification, which canhold stored data even when not powered, and which has an unlimitednumber of write cycles will be described with reference to drawings.

FIGS. 14A and 14B illustrate an example of a structure of thesemiconductor device. FIG. 14A is a cross-sectional view of thesemiconductor device, and FIG. 14B is a circuit diagram of thesemiconductor device.

The semiconductor device illustrated in FIGS. 14A and 14B includes atransistor 3200 including a first semiconductor material in a lowerportion, and a transistor 3202 including a second semiconductor materialin an upper portion. The structure of the transistor 440 described inEmbodiment 1 is applied to the transistor 3202.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial (such as silicon) other than a wide band gap semiconductor, andthe second semiconductor material can be a wide band gap semiconductor.A transistor including a material other than a wide band gapsemiconductor can operate at high speed easily. On the other hand, atransistor including a wide band gap semiconductor enables charge to beheld for a long time owing to its characteristics.

Although both of the above transistors are n-channel transistors in thefollowing description, it is needless to say that p-channel transistorscan be used. The specific structure of the semiconductor device, such asthe material used for the semiconductor device and the structure of thesemiconductor device, is not necessarily limited to those described hereexcept for the use of the transistor described in any of Embodiments 1to 4, which is formed using a wide band gap semiconductor for holdingdata.

The transistor 3200 in FIG. 14A includes a channel formation regionprovided in a substrate 3000 including a semiconductor material (such assilicon), impurity regions provided such that the channel formationregion is sandwiched therebetween, intermetallic compound regionsprovided in contact with the impurity regions, a gate insulating layerprovided over the channel formation region, and a gate electrode layerprovided over the gate insulating layer. Note that a transistor whosesource electrode layer and drain electrode layer are not illustrated ina drawing may be referred to as a transistor for the sake ofconvenience. Further, in such a case, in description of a connection ofa transistor, a source region and a source electrode layer may becollectively referred to as a source electrode layer, and a drain regionand a drain electrode layer may be collectively referred to as a drainelectrode layer. That is, in this specification, the term “sourceelectrode layer” may include a source region.

Further, an element isolation insulating layer 3106 is formed on thesubstrate 3000 so as to surround the transistor 3200, and an insulatinglayer 3220 is formed so as to cover the transistor 3200.

The transistor 3200 formed using a single crystal semiconductorsubstrate can operate at high speed. Thus, when the transistor is usedas a reading transistor, data can be read at a high speed. As treatmentprior to formation of the transistor 3202 and a capacitor 3204, CMPtreatment is performed on the insulating layer 3220 covering thetransistor 3200, whereby the insulating layer 3220 is planarized and, atthe same time, an upper surface of the gate electrode layer of thetransistor 3200 is exposed.

The transistor 3202 shown in FIG. 14A is a bottom-gate transistorincluding a wide band gap semiconductor in the channel formation region.Here, an oxide semiconductor layer included in the transistor 3202 ispreferably highly purified. By using a highly purified oxidesemiconductor layer, the transistor 3202 which has extremely favorableoff-state characteristics can be obtained.

FIG. 14B shows an example of a semiconductor memory device whichincludes the transistor 3202. When a transistor with low off-statecurrent is used as the transistor 3202, stored data can be held for along time in the semiconductor memory device. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationcan be extremely lowered, which leads to a sufficient reduction in powerconsumption.

One of source and drain electrode layers of the transistor 3202 iselectrically connected to an electrode 3208 through an opening providedin a gate insulating layer and is electrically connected to the gateelectrode layer of the transistor 3200 via the electrode 3208. Theelectrode 3208 can be formed through a process similar to that for agate electrode layer of the transistor 3202.

Insulating layers 3222, 3223, and 3223 a are provided over thetransistor 3202. In addition, a conductive layer 3210 a is provided in aregion overlapping with the one of the source and drain electrode layersof the transistor 3202 with the insulating layers 3222, 3223, and 3223 aprovided therebetween, and the one of the source and drain electrodelayers of the transistor 3202, the insulating layer 3222, and theconductive layer 3210 a form the capacitor 3204. That is, the one of thesource and drain electrode layers of the transistor 3202 functions asone electrode of the capacitor 3204, and the conductive layer 3210 afunctions as the other electrode of the capacitor 3204. Note that in thecase where no capacitor is needed, a structure in which the capacitor3204 is not provided is also possible. Alternatively, the capacitor 3204may be separately provided above the transistor 3202.

An insulating layer 3224 is provided over the capacitor 3204. Inaddition, a wiring 3216 for connecting the transistor 3202 to anothertransistor is provided over the insulating layer 3224. The wiring 3216is electrically connected to the other of the source and drain electrodelayers of the transistor 3202 through an electrode 3214 provided in anopening formed in the insulating layer 3224, a conductive layer 3210 bformed using the same layer as the conductive layer 3210 a, and anelectrode 3212 provided in an opening formed in the insulating layer3222.

In FIGS. 14A and 14B, the transistors 3200 and 3202 are provided so asto at least partly overlap each other, and the source region or thedrain region of the transistor 3200 is preferably provided to partlyoverlap with the oxide semiconductor layer included in the transistor3202. In addition, the transistor 3202 and the capacitor 3204 areprovided so as to overlap with at least part of the transistor 3200. Forexample, the conductive layer 3210 a of the capacitor 3204 is providedso as to at least partly overlap with the gate electrode layer of thetransistor 3200. When such a planar layout is employed, the areaoccupied by the semiconductor device can be reduced; thus, the degree ofintegration can be increased.

Next, an example of a circuit configuration corresponding to FIG. 14A isillustrated in FIG. 14B.

In FIG. 14B, a first wiring (1st Line) is electrically connected to asource electrode layer of the transistor 3200. A second wiring (2ndLine) is electrically connected to a drain electrode layer of thetransistor 3200. A third wiring (3rd Line) is electrically connected toone of the source and drain electrode layers of the transistor 3202, anda fourth wiring (4th Line) is electrically connected to the gateelectrode layer of the transistor 3202. The gate electrode layer of thetransistor 3200 and the other of the source and drain electrode layersof the transistor 3202 are electrically connected to one electrode ofthe capacitor 3204. A fifth wiring (5th Line) is electrically connectedto the other electrode of the capacitor 3204.

The semiconductor device in FIG. 14B utilizes a characteristic in whichthe potential of the gate electrode layer of the transistor 3200 can beheld, and thus enables data writing, holding, and reading as follows.

Writing and holding of data will be described. First, the potential ofthe fourth wiring is set to a potential at which the transistor 3202 isturned on, so that the transistor 3202 is turned on. Accordingly, thepotential of the third wiring is supplied to the gate electrode layer ofthe transistor 3200 and to the capacitor 3204. That is, predeterminedcharge is supplied to the gate electrode layer of the transistor 3200(writing). Here, one of two kinds of charges providing differentpotential levels (hereinafter referred to as a low-level charge and ahigh-level charge) is supplied. After that, the potential of the fourthwiring is set to a potential at which the transistor 3202 is turned off,so that the transistor 3202 is turned off. Thus, the charge supplied tothe gate electrode layer of the transistor 3200 is held (holding).

Since the off-state current of the transistor 3202 is significantlysmall, the charge of the gate electrode layer of the transistor 3200 isheld for a long time.

Next, reading of data will be described. By supplying an appropriatepotential (a reading potential) to the fifth wiring while supplying apredetermined potential (a constant potential) to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld at the gate electrode layer of the transistor 3200. This is becausein general, when the transistor 3200 is an n-channel transistor, anapparent threshold voltage V_(th) _(—) _(H) in the case where thehigh-level charge is given to the gate electrode layer of the transistor3200 is lower than an apparent threshold voltage V_(th) _(—) _(L) in thecase where the low-level charge is given to the gate electrode layer ofthe transistor 3200. Here, an apparent threshold voltage refers to thepotential of the fifth wiring which is needed to turn on the transistor3200. Thus, the potential of the fifth wiring is set to a potential V₀which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby chargesupplied to the gate electrode layer of the transistor 3200 can bedetermined. For example, in the case where the high-level charge issupplied in writing, when the potential of the fifth wiring is V₀(>V_(th) _(—) _(H)), the transistor 3200 is turned on. In the case wherethe low-level charge is supplied in writing, even when the potential ofthe fifth wiring is V₀ (<V_(th) _(—) _(L)), the transistor 3200 remainsoff. Therefore, the data held can be read by measuring the potential ofthe second wiring.

Note that in the case where memory cells are arrayed, it is necessarythat data of only a desired memory cell can be read. In that case, thefifth wirings of memory cells from which data is not read may besupplied with a potential at which the transistor 3200 is turned offregardless of the state of the gate electrode layer, that is, apotential lower than V_(th) _(—) _(H). Alternatively, the fifth wiringsmay be supplied with a potential at which the transistor 3200 is turnedon regardless of the state of the gate electrode layer, that is, apotential higher than V_(th) _(—) _(L).

When including a transistor having a channel formation region formedusing a wide band gap semiconductor and having extremely low off-statecurrent, the semiconductor device described in this embodiment can storedata for an extremely long period. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation can beextremely lowered, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be held for a long period evenwhen power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnonvolatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating layer does not arise at all. That is, thesemiconductor device according to the disclosed invention does not havea limitation on the number of times data can be rewritten, which is aproblem of a conventional nonvolatile memory, and the reliabilitythereof is drastically improved. Furthermore, since data are written byturning on or off the transistors, high-speed operation can be easilyachieved.

As described above, a miniaturized and highly integrated semiconductordevice having favorable electric characteristics and a method formanufacturing the semiconductor device can be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 7

In this embodiment, one embodiment of a structure of a memory devicewhich is different from that in Embodiment 6 will be described.

FIG. 15 is a perspective view of a memory device. The memory deviceillustrated in FIG. 15 includes a plurality of layers of memory cellarrays (memory cell arrays 3400(1) to 3400(n) (n is an integer greaterthan or equal to 2)) each including a plurality of memory cells asmemory circuits in the upper portion, and a logic circuit 3004 in thelower portion which is necessary for operating the memory cell arrays3400(1) to 3400(n).

FIG. 16 is a partial enlarged view of the memory device illustrated inFIG. 15. FIG. 16 illustrates the logic circuit 3004, the memory cellarray 3400(1), and the memory cell array 3400(2), and illustrates amemory cell 3170 a and a memory cell 3170 b as typical examples amongthe plurality of memory cells included in the memory cell array 3400(1)and the memory cell array 3400(2). The memory cell 3170 a and the memorycell 3170 b can have a configuration similar to the circuitconfiguration described in the above embodiment, for example.

Note that a transistor 3171 a included in the memory cell 3170 a isillustrated as a typical example. A transistor 3171 b included in thememory cell 3170 b is illustrated as a typical example. Each of thetransistors 3171 a and 3171 b includes a channel formation region in anoxide semiconductor layer. The structure of the transistor in which thechannel formation region is formed in the oxide semiconductor layer isthe same as the structure described in any of the other embodiments, andthus the description of the structure is omitted.

A conductive layer 3501 a which is formed using the same layer as a gateelectrode layer of the transistor 3171 a is electrically connected to anelectrode 3003 a via an electrode 3502 a. A conductive layer 3501 cwhich is formed using the same layer as a gate electrode layer of thetransistor 3171 b is electrically connected to an electrode 3003 c viaan electrode 3502 c.

The logic circuit 3004 includes a transistor 3001 in which asemiconductor material other than a wide band gap semiconductor is usedfor a channel formation region. The transistor 3001 can be a transistorobtained in such a manner that an element isolation insulating layer3106 is provided on a substrate 3000 including a semiconductor material(e.g., silicon) and a region serving as the channel formation region isformed in a region surrounded by the element isolation insulating layer3106. Note that the transistor 3001 may be a transistor obtained in sucha manner that the channel formation region is formed in a semiconductorfilm such as a silicon film formed on an insulating surface or in asilicon film of an SOI substrate. A known structure can be used as thestructure of the transistor 3001 and thus the description is omitted.

A wiring 3100 a and a wiring 3100 b are formed between layers in whichthe transistor 3171 a is formed and layers in which the transistor 3001is formed. An insulating film 3140 a is provided between the wiring 3100a and the layers in which the transistor 3001 is formed. An insulatingfilm 3141 a is provided between the wiring 3100 a and the wiring 3100 b.An insulating film 3142 a is provided between the wiring 3100 b and thelayers in which the transistor 3171 a is formed.

Similarly, a wiring 3100 c and a wiring 3100 d are formed between thelayers in which the transistor 3171 b is formed and the layers in whichthe transistor 3171 a is formed. An insulating film 3140 b is providedbetween the wiring 3100 c and the layers in which the transistor 3171 ais formed. An insulating film 3141 b is provided between the wiring 3100c and the wiring 3100 d. An insulating film 3142 b is provided betweenthe wiring 3100 d and the layers in which the transistor 3171 b isformed.

The insulating films 3140 a, 3141 a, 3142 a, 3140 b, 3141 b, and 3142 beach function as an interlayer insulating film whose surface can beplanarized.

The wirings 3100 a, 3100 b, 3100 c, and 3100 d enable electricalconnection between the memory cells, electrical connection between thelogic circuit 3004 and the memory cells, and the like.

An electrode 3303 included in the logic circuit 3004 can be electricallyconnected to a circuit provided in the upper portion.

For example, as illustrated in FIG. 16, the electrode 3303 can beelectrically connected to the wiring 3100 a via an electrode 3505. Thewiring 3100 a can be electrically connected to a conductive layer 3501 bwhich is formed using the same layer as the gate electrode layer of thetransistor 3171 a via an electrode 3503 a. In this manner, the wiring3100 a and the electrode 3303 can be electrically connected to thesource or the drain of the transistor 3171 a. The conductive layer 3501b can be electrically connected to an electrode 3003 b via the source orthe drain of the transistor 3171 a and an electrode 3502 b. Theelectrode 3003 b can be electrically connected to the wiring 3100 c viaan electrode 3503 b.

FIG. 16 illustrates an example in which the electrode 3303 and thetransistor 3171 a are electrically connected to each other via thewiring 3100 a; however, one embodiment of the disclosed invention is notlimited thereto. The electrode 3303 may be electrically connected to thetransistor 3171 a via the wiring 3100 b, via both the wiring 3100 a andthe wiring 3100 b, or via another electrode without using the wiring3100 a nor the wiring 3100 b.

FIG. 16 illustrates the structure where two wiring layers, i.e., awiring layer in which the wiring 3100 a is formed and a wiring layer inwhich the wiring 3100 b is formed are provided between the layers inwhich the transistor 3171 a is formed and the layers in which thetransistor 3001 is formed; however, the number of wiring layers providedtherebetween is not limited to two. One wiring layer or three or morewiring layers may be provided between the layers in which the transistor3171 a is formed and the layers in which the transistor 3001 is formed.

FIG. 16 illustrates the structure where two wiring layers, i.e., awiring layer in which the wiring 3100 c is formed and a wiring layer inwhich the wiring 3100 d is formed are provided between the layers inwhich the transistor 3171 b is formed and the layers in which thetransistor 3171 a is formed; however, the number of wiring layersprovided therebetween is not limited to two. One wiring layer or threeor more wiring layers may be provided between the layers in which thetransistor 3171 b is formed and the layers in which the transistor 3171a is formed.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 8

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices include a television device (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone, a portable game machine, a portableinformation terminal, an audio reproducing device, a game machine (suchas a pachinko machine or a slot machine), a game console, and the like.FIGS. 17A to 17C illustrate a specific example of an electronic device.

FIGS. 17A and 17B illustrate a tablet terminal that can be folded. InFIG. 17A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of Embodiments 1 to 3 can beused for the display portion 9631 a and the display portion 9631 b,whereby the tablet terminal can be provided with high reliability.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. The whole region in the display portion 9631 amay have a touch panel function. For example, the display portion 9631 acan display a keyboard in the whole region to be used as a touch panel,and the display portion 9631 b can be used as a display screen.

Like the display portion 9631 a, a touch panel region 9632 b can beformed in part of the display portion 9631 b. When a keyboard displayswitching button 9639 displayed on the touch panel is touched with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. With the power-saving-mode switching button 9036,the luminance of display can be optimized in accordance with the amountof external light at the time when the tablet is in use, which isdetected with an optical sensor incorporated in the tablet. The tabletterminal may include another detection device such as a sensor fordetecting inclination (e.g., a gyroscope or an acceleration sensor) inaddition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 17A, one embodiment of the presentinvention is not limited to this structure. The display portion 9631 aand the display portion 9631 b may have different areas or differentdisplay quality. For example, one of them may be a display panel thatcan display higher-definition images than the other.

In FIG. 17B, the tablet terminal is folded, and includes the housing9630, a solar battery 9633, a charge and discharge control circuit 9634,a battery 9635, and a DCDC converter 9636. FIG. 17B illustrates anexample in which the charge and discharge control circuit 9634 includesthe battery 9635 and the DCDC converter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen the tablet terminal is not used. Thus, the display portions 9631 aand 9631 b can be protected, thereby providing the tablet terminal withhigh endurance and high reliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 17A and 17B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630, sothat the battery 9635 can be charged efficiently. The use of a lithiumion battery as the battery 9635 is advantageous in downsizing or thelike.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 17B are described with reference to a blockdiagram of FIG. 17C. FIG. 17C illustrates the solar battery 9633, thebattery 9635, the DCDC converter 9636, a converter 9637, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 17B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that the power has a voltage for charging thebattery 9635. When the display portion 9631 is operated with the powerfrom the solar battery 9633, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to a voltageneeded for operating the display portion 9631. In addition, when displayon the display portion 9631 is not performed, the switch SW1 is turnedoff and a switch SW2 is turned on so that charge of the battery 9635 maybe performed.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

EXPLANATION OF REFERENCE

-   400: substrate, 401: gate electrode layer, 402: gate insulating    layer, 403: oxide semiconductor layer, 405: low-resistance material    layer, 405 a: low-resistance material layer, 405 b: low-resistance    material layer, 406: first protective layer, 407: second protective    layer, 407 a: third protective layer, 407 b: fourth protective    layer, 420: transistor, 421: transistor, 430: capacitor, 431:    transistor, 432: insulating layer, 436: base insulating layer, 440:    transistor, 441: transistor, 451: oxygen doping, 453: resist, 454:    conductive layer, 454 a: first conductive layer, 454 b: second    conductive layer, 455: resist, 474 a: wiring layer, 474 b: wiring    layer, 485: wiring protective layer, 485 a: first wiring protective    layer, 485 b: second wiring protective layer, 495: hard mask layer,    3000: substrate, 3001: transistor, 3003 a: electrode, 3003 b:    electrode, 3003 c: electrode, 3004: logic circuit, 3100 a: wiring,    3100 b: wiring, 3100 c: wiring, 3100 d: wiring, 3106: element    isolation insulating layer, 3140 a: insulating film, 3140 b:    insulating film, 3141 a: insulating film, 3141 b: insulating film,    3142 a: insulating film, 3142 b: insulating film, 3170 a: memory    cell, 3170 b: memory cell, 3171 a: transistor, 3171 b: transistor,    3200: transistor, 3202: transistor, 3204: capacitor, 3208:    electrode, 3210 a: conductive layer, 3210 b: conductive layer, 3212:    electrode, 3214: electrode, 3216: wiring, 3220: insulating layer,    3222: insulating layer, 3223: insulating layer, 3223 a: insulating    layer, 3224: insulating layer, 3303: electrode, 3400(1): memory cell    array, 3400(2): memory cell array, 3400(n): memory cell array, 3501    a: conductive layer, 3501 b: conductive layer, 3501 c: conductive    layer, 3502 a: electrode, 3502 b: electrode, 3502 c: electrode, 3503    a: electrode, 3503 b: electrode, 3505: electrode, 9033: clip, 9034:    button, 9035: power button, 9036: button, 9038: operation button,    9630: housing, 9631: display portion, 9631 a: display portion, 9631    b: display portion, 9632 a: region, 9632 b: region, 9633: solar    battery, 9634: charge and discharge control circuit, 9635: battery,    9636: DCDC converter, 9637: converter, 9638: operation key, 9639:    button.

This application is based on Japanese Patent Application serial no.2011-282438 filed with Japan Patent Office on Dec. 23, 2011 and JapanesePatent Application serial no. 2011-282511 filed with Japan Patent Officeon Dec. 23, 2011, the entire contents of which are hereby incorporatedby reference.

1. A method for manufacturing a semiconductor device, comprising thesteps of: forming a gate electrode layer; forming a gate insulatinglayer over the gate electrode layer; forming a semiconductor layer overthe gate insulating layer; forming a first conductive layer over thesemiconductor layer; forming a second conductive layer over the firstconductive layer so that the first conductive layer is exposed in aregion which overlaps with a channel formation region of thesemiconductor layer; forming a protective layer over the secondconductive layer; forming a hard mask layer over the protective layer;forming a resist pattern over the hard mask layer, the resist patternincluding an opening which overlaps with the semiconductor layer;etching the hard mask layer using the resist pattern; etching theprotective layer using the hard mask layer; and etching the firstconductive layer using the hard mask layer.
 2. The method formanufacturing the semiconductor device according to claim 1, wherein thesemiconductor layer includes an oxide including In, an element M, andZn, and wherein the element M is at least one element selected from thegroup consisting of Ga, Sn, Hf, Al, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, and Lu.
 3. The method for manufacturing thesemiconductor device according to claim 1, wherein the opening in theresist pattern is formed by an electron beam.
 4. The method formanufacturing the semiconductor device according to claim 1, wherein thehard mask layer comprises amorphous silicon.
 5. The method formanufacturing the semiconductor device according to claim 1, wherein achannel length of a transistor including the semiconductor layer isgreater than or equal to 20 nm and less than or equal to 100 nm.
 6. Themethod for manufacturing the semiconductor device according to claim 1,wherein a length of the semiconductor layer in a channel lengthdirection is larger than a length of the gate electrode layer in thechannel length direction.
 7. The method for manufacturing thesemiconductor device according to claim 1, further comprising the stepof planarizing the gate insulating layer before forming thesemiconductor layer.
 8. The method for manufacturing the semiconductordevice according to claim 1, further comprising the step of planarizingthe protective layer before forming the hard mask layer.
 9. A method formanufacturing a semiconductor device, comprising the steps of: forming agate electrode layer; forming a gate insulating layer over the gateelectrode layer; forming a semiconductor layer over the gate insulatinglayer; forming a first conductive layer over the semiconductor layer;forming a second conductive layer over the first conductive layer;forming a protective layer over the second conductive layer; forming afirst resist pattern over the protective layer, the first resist patternincluding a first opening; etching the protective layer and the secondconductive layer using the first resist pattern; forming, after the stepof etching, a second resist pattern over the first conductive layer andthe protective layer, the second resist pattern including a secondopening in a region which overlaps with the semiconductor layer; andetching the first conductive layer using the second resist pattern. 10.The method for manufacturing the semiconductor device according to claim9, wherein the semiconductor layer includes an oxide including In, anelement M, and Zn, and wherein the element M is at least one elementselected from the group consisting of Ga, Sn, Hf, Al, Zr, La, Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
 11. The method formanufacturing the semiconductor device according to claim 9, wherein thesecond opening is formed by an electron beam.
 12. The method formanufacturing the semiconductor device according to claim 9, wherein achannel length of a transistor including the semiconductor layer isgreater than or equal to 20 nm and less than or equal to 100 nm.
 13. Themethod for manufacturing the semiconductor device according to claim 9,wherein a length of the semiconductor layer in a channel lengthdirection is larger than a length of the gate electrode layer in thechannel length direction.
 14. The method for manufacturing thesemiconductor device according to claim 9, further comprising the stepof planarizing the gate insulating layer before forming thesemiconductor layer.
 15. A method for manufacturing a semiconductordevice, comprising the steps of: forming an oxide semiconductor layer;forming a first conductive layer over the oxide semiconductor layer;forming a second conductive layer over the first conductive layer sothat the first conductive layer is exposed in a region which overlapswith a channel formation region of the oxide semiconductor layer;forming a protective layer over the second conductive layer; forming ahard mask layer over the protective layer; forming a resist pattern overthe hard mask layer, the resist pattern including an opening whichoverlaps with the oxide semiconductor layer; etching the hard mask layerusing the resist pattern; etching the protective layer using the hardmask layer; and etching the first conductive layer using the hard masklayer.
 16. A semiconductor device comprising: a gate electrode layer; agate insulating layer on one surface of the gate electrode layer; asemiconductor layer on one surface of the gate insulating layer; a firstconductive layer over the semiconductor layer; a second conductive layerover the first conductive layer; a protective layer over the secondconductive layer; and a hard mask layer over the protective layer,wherein the hard mask layer comprises an opening which overlaps with achannel formation region of the semiconductor layer.
 17. Thesemiconductor device according to claim 16, wherein the semiconductorlayer includes an oxide including In, an element M, and Zn, and whereinthe element M is at least one element selected from the group consistingof Ga, Sn, Hf, Al, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, and Lu.
 18. The semiconductor device according to claim 16, whereinthe hard mask layer comprises amorphous silicon.
 19. The semiconductordevice according to claim 16, wherein a thickness of the semiconductorlayer is greater than or equal to 5 nm and less than or equal to 30 nm.20. The semiconductor device according to claim 16, wherein a channellength of a transistor including the semiconductor layer is greater thanor equal to 20 nm and less than or equal to 100 nm.
 21. Thesemiconductor device according to claim 16, wherein a length of thesemiconductor layer in a channel length direction is larger than alength of the gate electrode layer in the channel length direction. 22.The semiconductor device according to claim 16, wherein a width of theopening is substantially the same as a length of the semiconductor layerin a channel length direction.
 23. The semiconductor device according toclaim 16, wherein the semiconductor layer is located over the gateelectrode layer.
 24. A semiconductor device comprising: a gate electrodelayer; a gate insulating layer on one surface of the gate electrodelayer; a semiconductor layer on one surface of the gate insulatinglayer; a first conductive layer and a second conductive layer over thesemiconductor layer; a third conductive layer over the first conductivelayer; a fourth conductive layer over the second conductive layer; and aprotective layer over the third conductive layer and the fourthconductive layer, wherein a distance between the first conductive layerand the second conductive layer is shorter than a distance between thethird conductive layer and the fourth conductive layer, and wherein thefirst conductive layer and the third conductive layer serve as a sourceelectrode and the second conductive layer and the fourth conductivelayer serve as a drain electrode.
 25. The semiconductor device accordingto claim 24, wherein the semiconductor layer includes an oxide includingIn, an element M, and Zn, and wherein the element M is at least oneelement selected from the group consisting of Ga, Sn, Hf, Al, Zr, La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
 26. Thesemiconductor device according to claim 24, wherein a thickness of thesemiconductor layer is greater than or equal to 5 nm and less than orequal to 30 nm.
 27. The semiconductor device according to claim 24,wherein a channel length of a transistor including the semiconductorlayer is greater than or equal to 20 nm and less than or equal to 100nm.
 28. The semiconductor device according to claim 24, wherein a lengthof the semiconductor layer in a channel length direction is larger thana length of the gate electrode layer in the channel length direction.29. The semiconductor device according to claim 24, wherein thesemiconductor layer is located over the gate electrode layer.